Problem with OCD V3 clone

[DaanJM]

Hey,

I just build the third version of the Fulltone OCD (vero layout from http://tagboardeffects.blogspot.nl/2012/02/fulltone-ocd-all-versions.html).
The sound is quite good, until I use a buffered pedal like the Boss DD-5 (or the buffer I made myself) after the OCD. Suddenly the volume and treble boost a bit. Apparently it only happens when the volume pot is not on 10. I have no problem at all when the volume is maxed.

Does this have something to do with the output impedance of the pedal? Maybe the value of the volume pot (500k) is too high?

Any help would be greatly appreciated.